I think the Voltage reg. kicks in as soon as line voltage drops to 112 volts. Regulated voltage is 120 volts +/- 5% I think but ive never seen it go beyond 122 and it holds 120 pretty well unless voltage really drops out a bit below 110 volts. I believe that it kicks back off at 117 volts.
Anyway, I just tried a .75 ohm and .6 ohm. Line voltage was up at 118 volts (voltage reg. off) and heater voltage was 5.98 volts. That was with the .6 ohm resistor so I think it's probably a good choice.
Paul J. suggests .68 ohms at 115 volts and .91 ohms at 120 volts as starting values with 6DJ8. .6 seems close enough at 118 volts. I think heater voltage should stay within 5.7 - 6.3 volts with line voltages from 113 to 122 Volts. With the .75 I think it may drop a little below 5.7 if line voltage dips to 114 or 113 volts but either would probably work and I dont have anything in the middle.
I think it'd probably be splitting hairs anyway. I probably am already